1. Field of the Invention
Embodiments of the invention relate to a display device, and more particularly, to an apparatus and a method of driving a backlight of a liquid crystal display. Although embodiments of the invention are suitable for a wide scope of applications, they are particularly suitable for minimizing wave noise on a liquid crystal panel generated by a backlight that uses light emitting diodes (LEDs) as a light source.
2. Description of the Related Art
In general, the application of liquid crystal displays (hereinafter, simply referred to as “LCDs”) has extended into office automation equipment, audio/video devices and the like due to characteristics, such as light weight, small size, and low power consumption. The LCDs are devices that display desired images by controlling transmittance of light generated from a backlight according to image signals that are applied to a plurality of control switches arranged in a matrix shape.
LCDs are not self-luminous displays, and thus each of the LCDs includes a light source, such as a backlight, that is disposed at the rear of the LCD. In general, fluorescent lamps are used as the backlight of the LCD. Light sources for an LCD are divided into a direct type LCD and a side type LCD according to the position of the backlight. Light emitting diodes (LEDs) have been widely used as backlights of small LCDs in personal digital assistants (PDAs), cellular phones, notebook computers and the like.
FIGS. 1(a) and 1(b) are perspective views illustrating a structure of a side type backlight and a structure of a direct type backlight, respectively. More specifically, FIG. 1(a) is a view illustrating a structure of a side type backlight in which sets of light emitting diode arrays 12 are formed at sides of a diffuse film lining cavity 11. Further, FIG. 1(b) is a view illustrating a structure of a direct type backlight in which the sets of light emitting diode arrays 12 are formed at a rear surface of the diffuse film lining cavity 11.
FIGS. 2 and 3 illustrate arrangements of light emitting diodes that are used as the backlight in the liquid crystal display according to the related art. FIGS. 2(a) and 2(b) are schematic views each illustrating a backlight that is implemented with a few high-power light emitting diodes. FIG. 3 is a schematic view illustrating a backlight that is implemented with normal light emitting diodes. FIG. 4 is a block diagram of a driving circuit of a backlight according to the related art.
As shown in FIG. 4, an apparatus for driving a backlight includes red, green and blue light emitting diode driving units 41R, 41G, and 41B that drive red, green and blue light emitting diode arrays 42R, 42G, and 42B, respectively. More specifically, the red, green and blue light emitting diode arrays 42R, 42G, and 42B are lit by pulse width modulation signals supplied from the red, green and blue light emitting diode driving units 41R, 41G, and 41B, respectively, so as to emit red, green, and blue light. The operation of the apparatus for driving a backlight that has the above-described structure will be described with reference to FIGS. 5(a), 5(b) and 5(c).
FIGS. 5(a), 5(b), and 5(c) are waveforms of pulse width modulation signals for red, green, and blue, respectively. The red, green and blue light emitting diode driving units 41R, 41G, and 41B respectively drive in a burst mode, red, green and blue light emitting diode arrays 42R, 42G, and 42B in which light emitting diodes LED_R, green light emitting diodes LED_G, and blue light emitting diodes LED_B are connected in series in their respective array. Further, the red, green and blue light emitting diode driving units 41R, 41G, and 41B perform dimming control in the burst mode with a pulse width modulation signal PWM_R, PWM_G, and PWM_B for red, green, and blue lights, as shown in FIGS. 5(a), 5(b), and 5(c).
When the red, green, and blue light emitting diode driving units 41R, 41G, and 41B output the pulse width modulation signals PWM_R, PWM_G, and PWM_B to the red, green, and blue light emitting diode arrays 42R, 42G, and 42B, the pulse width modulation signal PWM_R, PWM_G, and PWM_B for red, green, and blue are synchronized and output, as shown in FIGS. 5(a), 5(b), and 5(c). Therefore, the red light emitting diodes LED_R, the green light emitting diodes LED_G, and the blue light emitting diodes LED_B are lit by the pulse width modulation signal PWM_R, PWM_G, and PWM_B that are respectively supplied from the red, green, and blue light emitting diode driving units 41R, 41G, and 41B, such that red, green, and blue light components are transmitted. The red, green, and blue light components mix to produce white light, which is supplied toward the rear surface of the liquid crystal panel.
As shown in FIGS. 5(a), 5(b) and 5(c), the pulse width modulation signals PWM_R, PWM_G, and PWM_B have periods where the signals overlap each other. When the three pulse width modulation signals PWM_R, PWM_G, and PWM_B for red, green, and blue light overlap each other, frequencies generated from the red, green, and blue light emitting diode driving units 41R, 41G, and 41B affect data lines of the liquid crystal panel, which causes distortion in the charging time of the data lines. Wave noise is generated in the liquid crystal panel due to distortion in the charging time of the data lines. To prevent the wave noise from occurring in the liquid crystal panel, a method that changes PWM dimming frequencies of the red, green, and blue light emitting diode driving units 41R, 41G, and 41B has been used. However, this method of removing the wave noise is fundamentally difficult to implement. Further, since frequency margins are small, it is difficult to efficiently prevent the generation of the wave noise.